1. Field of the Invention
This invention relates to a tape-shaped optical fiber cable and, in particular, to a tape-shaped optical fiber cable that is used as a fiber sensor etc. to be installed in a structure so as to measure its temperature, strain etc.
2. Description of the Related Art
A tape-shaped optical fiber cable is known which is installed in a structure as a fiber sensor in order to measure its temperature and strain (e.g., See nonpatent literatures 1 and 2 as mentioned later).
As shown in FIG. 10, the tape-shaped optical fiber cable 70 has a tape member 73 formed by embedding unidirectional-arrayed glass fibers 71 in a thermoplastic resin 72, and a covered optical fiber 76 embedded in the tape member 73. The covered optical fiber 76 has an optical fiber 74 and a covering material 75 of polyimide etc. covering the optical fiber 74. For example, the tape-shaped optical fiber cable 70 is 13 mm in width and 0.2 mm in thickness, the optical fiber 74 is 125 μm in diameter, and the covered optical fiber 76 is 145 μm in diameter.
The tape-shaped optical fiber cable 70 is installed in a structure by being fixed to a surface thereof or by being embedded in an inside thereof in order to measure a strain applied to the structure itself. Therefore, the tape member 73 uses the thermoplastic resin 72 and the glass fiber 71 with an excellent heat resistance so as to measure the strain even in a high-temperature environment such as a plant. The tape member 73 is formed by heating the thermoplastic resin to a predetermined temperature to soften the resin, and then molding it at a low temperature. An optical fiber generally used for communications has a low heat resistance, i.e., its heat resistance temperature is not more than 100° C., since it is covered by a covering material of ultraviolet curable acrylic resin. Therefore, in the tape-shaped optical fiber cable 70, a polyimide covering material with a heat resistance temperature of about 300° C. is adopted as the heat-resistant covering material for the fiber sensor.
The tape-shaped optical fiber cable 70 is used as a sensing part of the fiber sensor such that it can measure a change in a measured object by detecting a reflected light (a backscattered light) propagating through the optical fiber.
FIG. 11 is a graph showing a backscattered light spectrum. As shown in FIG. 11, for example, when a Rayleigh scattering light RL with a central wavelength λ0 is generated, Brillouin scattering lights Bs, Ba are respectively generated at the long-wavelength side and the short-wavelength side of the λ0. Further, Raman scattering lights RMa, RMs are respectively generated at the shorter-wavelength side of the short-wavelength Brillouin scattering light Ba, and at the longer-wavelength side of the long-wavelength Brillouin scattering light Bs. The Raman scattering light RMs is also called a Stokes light and the Raman scattering lights RMa is also called an anti-Stokes light.
In case of using the optical fiber as a strain measuring sensor, amount of a wavelength shift in the Brillouin scattering light Bs is measured to determine the strain based on the measured value. On the other hand, in case of using the optical fiber as a temperature measuring sensor, temperature is determined based on the intensity ratio of the Stokes light RMs and the anti-Stokes light RMa in the Raman scattering light.
The following patent literature and nonpatent literatures are assumed relevant to the tape-shaped optical fiber cable of the invention.
Patent literature 1: JP-A-2002-70015
Nonpatent literature 1: B. Glisic, D. Inaudi, “Sensing tape for easy integration of optical fiber sensors in composite structures” SPIE, International Symposium on Smart Structures and Materials, 2-6, Mar. 2003.
Nonpatent literature 2: B. Glisic, D. Inaudi, “Integration of long-gage fiber-optic sensor into a fiber-reinforced composite sensing tape” 16th International Conference on Optical Fiber Sensors, Nara, Japan, 13-17, Oct. 2003.
However, the following problems are found in the conventional tape-shaped optical fiber cable.
(1) The rigidity (tape rigidity) required for the tape-shaped optical fiber cable is different depending on the structure where the cable is installed. For example, the required rigidity is determined depending on environment where the cable is installed, e.g., a situation where it is installed in bent state and a situation where it is embedded in the structure.
The tape rigidity is determined by a thickness of the tape member 73. Further, the thickness of the tape member 73 can be determined depending on an outer diameter of the covered optical fiber 76, a thickness of the glass fiber 71, and a thickness of the thermoplastic resin 72.
The conventional tape-shaped optical fiber cable 70 is about 0.2 mm in tape thickness, and the covered optical fiber 76 is 145 μm in outer diameter. Thus, since the outer diameter of the covered optical fiber 76 is large relative to the tape thickness, it is difficult to reduce the tape thickness of the tape-shaped optical fiber cable to less than 0.2 mm. Therefore, the conventional tape-shaped optical fiber cable 70 may not be installed due to the installation environment such as a situation where the thickness of the cable 70 is limited by the shape of the structure to install the cable 70.
For example, when the tape-shaped optical fiber cable is embedded in a fuel tank or an aircraft formed of a carbon fiber reinforced plastic multilayer board (with a single-layer thickness of about 0.13 mm), it needs to have a tape thickness less than 0.13 mm, i.e., the single-layer thickness of the carbon fiber reinforced plastic multilayer board since it may cause a deterioration in strength by being embedded therein.
(2) Generally, a glass fiber is strong to tensile strength but is weak to shear strength. Since the conventional tape-shaped optical fiber cable 70 uses the unidirectional-arrayed glass fiber 71, the cable 70 is strong in the longitudinal direction of the fiber but is weak in a direction perpendicular to the longitudinal direction of the fiber. Thus, the cable 70 is suited to be installed in one direction (linearly) based on the tape rigidity, but it is not suited to be installed in bent state since the cable 70 may be broken due to a force applied from the side directions of the cable 70.
(3) The conventional tape-shaped optical fiber cable 70 uses a general optical fiber for communications. The optical fiber for communications is suitable for a long-distance transmission since it has a low light transmission loss in a situation where a macro-bending or a micro-bending is not caused.
However, since the optical fiber 74 is embedded in the tape member 73 and disposed between the glass fibers, the cable 70 may be subjected to a micro-bending generated due to a minute bending distributed in the longitudinal direction of the fiber or a compression strain applied to the cable 70 when the thermoplastic resin 72 is shrunk at a low temperature. Thus, the conventional tape-shaped optical fiber cable 70 will be subjected to a large light transmission loss caused by the macro-bending or the micro-bending. As a result, it becomes difficult to use the cable 70 to measure the strain over long distances.
(4) The conventional tape-shaped optical fiber cable 70 is used to measure the strain of a measured object where the cable 70 is installed by using the Brillouin scattering light whose amount of wavelength shift changes according to the stain applied to the optical fiber 74, and to measure the temperature of the measured object by using the Raman scattering light whose intensity ratio changes according to its temperature.
However, since the optical fiber 74 is subjected to a distributed bending loss caused by that the optical fiber cable is tape-shaped, it is difficult to distinguish a change in light intensity caused by the temperature from a change in light intensity caused by on the bending loss. As a result, it is difficult to accurately measure the temperature.